Temperature is measured by thermometer on physical properties that change linearly with temperature to give temperature readings. Physical properties of a thermometer include;
i)
Expansion of liquid (mercury
or alcohol) when heated
ii) Expansion of a compound strip of two metals
iii) Thermoelectric property change in which when junction of two
different metals are heated an electric current is generated
iv) Change of resistance of a wire e.g. platinum resistance thermometer
NB:
i)
The liquid used in
thermometer is called thermometric liquid
ii) Temperature of the body is measured in degree centigrade,
°C
Thermometric Scales
There are two scales of thermometer include;
i)
Celsius or Centigrade Scale
ii) Fahrenheit Scale
Celsius or Centigrade Scale
This scale has 100 divisions between the upper and lower
standard points. This scale was introduced by a Swedish Astronomer Celsius and
is known after his name. Each division on this scale is called one degree
centigrade or one degree Celsius and is written as 1°C.
More sensitive thermometers have 200 divisions between standard points and each division is equal to 1/2 °C. Sometimes these thermometers are called half °C thermometers.
Fahrenheit Scale
This scale was introduced by Fahrenheit. On this
scale 32°F represents the melting point of ice and 212°F the steam point. Zero
is marked 32°F below the ice point. The length in between the standard points is
divided to 180 equal parts. Each division on this scale is called 1°F. This
scale is widely used for meteorological and clinical purposes.
Diagram show the thermometric scale
Types of Thermometer
i)
Mercury in glass
thermometer
ii)
Alcohol in glass
thermometer
iii)
maximum and minimum
thermometer
iv)
thermocouple thermometer
v)
Thermistor thermometer
Liquid in Glass Thermometer
The working of this kind of thermometer is contraction
and expansion of the liquid inside the thermometer. Always mercury and alcohol
used as liquid inside thermometer so we have two types of liquid in glass
thermometer
i)
Mercury in glass
thermometer
ii)
Alcohol in glass
thermometer
Choice of Thermometric Liquid
A thermometric liquid must have the following properties
i)
It should have low specific
heat capacity, so that it rapidly attains the temperature of a given substance,
without absorbing any appreciable amount of heat energy from it.
ii) It should have a uniform rate of expansion, such that a linear
scale can be easy marked.
iii) It should have large expansion for a unit degree rise in
temperature, so that its expansion is visible to the unaided eye.
iv) It should have a high boiling point and low freezing point, so
that a wide range of temperature changes could be recorded by a single
thermometer.
v) It should be shiny and opaque so that it is clearly visible in
glass.
vi) It should not stick to the sides of the glass tube.
vii) It should exert low vapour pressure.
viii) It should be a good conductor of heat.
ix) It should be easily available in pure state.
Why Mercury Used in Thermometer
i)
It has low specific heat
capacity.
ii)
Its expansion is uniform.
iii)
It has a high boiling point
(357°C) and low melting point (-39°C).
iv)
It is opaque and shining.
v)
It does not stick to the
sides of the glass.
vi)
It exerts very low vapour
pressure.
vii)
It is a good conductor of
heat.
viii) It is easily available in pure state.
Disadvantages of Mercury as Thermometric Liquid
i)
Its expansion is not very
large for 1°C rise in temperature and hence, very small changes in temperature
cannot be measured.
ii) It freezes below -39°C and hence, it cannot be used in very cold regions like Antarctic or Arctic.
Why Alcohol Used in Thermometer
i)
Its freezing point is below
-100°C and hence, can record very low temperatures.
ii) Its expansion per degree centigrade rise in temperature is very
large and hence, very sensitive thermometers can be made with it.
iii) It can be coloured brightly and hence is easily visible.
Disadvantages of Alcohol as Thermometric Liquid
i)
It cannot be used for
measuring high temperatures as alcohol boils at 78°C.
ii) It has high specific heat capacity.
iii) It sticks to the sides of glass.
iv) It has a high vapour pressure.
v) It is not a good conductor of heat.
vi) It is difficult to obtain pure alcohol.
Disadvantages of Water as Thermometric Liquid
i)
It has the highest specific
heat capacity (4.2 J/gK).
ii) Its expansion is not uniform.
iii) Its expansion per degree rise in temperature is very small
iv) Its melting point is 0°C and boiling point 100°C. Thus, the
temperatures less than 0°C and more than 100°C cannot t measured.
v) It is transparent/colourless
vi) It sticks to the sides of glass.
vii) It evaporates under vacuum conditions.
viii) It is a bad conductor of heat.
ix) It cannot be obtained in cent per cent pure form easily.
Thermometer Fundamental Interval
Fundamental interval of a thermometer is the
different between the upper fixed point and the lower fixed point of the
thermometer.
Example, the upper fixed point of water is 100 °C and the lower fixed point of water is 0 °C
Types of Thermometer
vi)
Mercury in glass
thermometer
vii)
Alcohol in glass
thermometer
viii) maximum and minimum thermometer
ix)
thermocouple thermometer
x) Thermistor thermometer
Liquid in Glass Thermometer
The working of this kind of thermometer is contraction
and expansion of the liquid inside the thermometer. Always mercury and alcohol
used as liquid inside thermometer so we have two types of liquid in glass
thermometer
iii)
Mercury in glass
thermometer
iv)
Alcohol in glass
thermometer
No comments:
Post a Comment